Since no one can predict how long Microsoft will keep the patches for Windows XP in their update catalog file wsusscn2.cab after April 8th, 2014, I suggest to perform the following steps in order to keep XP update capability within WOU:

Soon after April 8th, 2014, execute a download run for Windows XP using the most recent version of WOU,

create an ISO image and

keep it!

Of course the catalog file inside this ISO Image will then be outdated when you install the patches later, but this doesn't matter anyway, because there won't be any new patches for Windows XP after April 8th, 2014.